Many multi-cylinder internal combustion engines include an evaporative fuel recovery system, in which fuel vapors vented from the fuel tank and captured in a carbon canister are drawn into the engine, where they are combusted along with fuel delivered by fuel injectors. Such systems can include a purge control valve, which controls the flow rate of canister purge fuel vapors entering the engine. Some purge control valves are on-off, pulse-width modulated valves, which are designed to be either fully open or fully closed. Pulse-width modulated valves can be driven by an electrical input signal which is high for a fraction of the signal period and low for the remainder of the signal period. The high portion of the signal is called the on-pulse. The valve opens to allow purge fuel vapors to enter the engine during the on-pulse and closes for the remainder of the signal period. The frequency and duration of the on-pulse determines the average flow rate through the valve.
If the purge control valve input signal frequency is kept at a constant value, there will be an engine speed, called the critical rpm value, at which the on-pulses align in time with the intake stroke of the same engine cylinder for many consecutive intake strokes. Such a situation can cause that particular cylinder to receive most of the purge fuel, while the other cylinders receive substantially less purge fuel. This is undesirable, because it can result in an excessively rich air to fuel ratio in one particular cylinder.
U.S. Pat. No. 5,682,863 (the '863 patent) proposes one approach for preventing the on-pulse of the purge control valve from aligning in time with the intake stroke of a particular cylinder. In particular, according to the '863 patent, the frequency of the purge control valve is continuously adjusted in response to changing engine speeds to avoid such alignments. The frequency of the purge control valve for a given engine speed is adjusted, during engine operation, so as to prevent the on-pulse of the purge control from aligning in time with the intake stroke of any particular cylinder. When engine speed changes, the frequency of the purge control valve is adjusted to avoid alignment at the new engine speed. This process occurs over and over again, during engine operation, because the engine speed repeatedly changes.
U.S. Pat. No. 5,429,098 (the '098 patent) also proposes changing the on-pulse frequency in response to changing engine speed. The '098 patent also proposes another approach for preventing the on-pulse of the purge control valve from aligning in time with the intake stroke of a particular cylinder. In particular, the '098 patent teaches changing the on-pulse frequency based on an elapsed time, which is measured with a timer. Using this process, the elapsed time is constantly monitored, and an on-pulse frequency that corresponds to a particular elapsed time is selected over and over again, during engine operation.
The inventor herein has recognized that the approaches disclosed in the '863 patent and the '098 patent have several issues. In particular, the '863 patent requires the purge control valve to change frequency, during engine operation, in response to changes in engine speed, and the '098 patent requires the purge control valve to change frequency, during engine operation, in response to changes in either engine speed or a measured elapsed time. Such approaches require the engine speed and/or an elapsed time to be monitored during engine operation. Furthermore, the '863 patent and the '098 patent require synchronization with engine cylinder events. Degradation in monitoring engine speed, monitoring an elapsed time, and/or synchronizing with engine cylinder events can cause the approaches of the '863 patent and the '098 patent to give erroneous results.
At least some of the above issues may be addressed by a system and method for changing purge valve on-pulse period (frequency) according to a predetermined schedule without monitoring engine speed, an elapsed time, or other real-time operating parameters of the engine. In this way, it may be possible to limit alignment between the on-pulse of a purge control valve and consecutive intake strokes of a particular cylinder without requiring synchronization with engine cylinder events, and/or real-time monitoring of engine speed, elapsed time, or other operating parameters of the engine.